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<title>PTC's Internal State </title>
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<p align="center"><font size="7" color="#FF00FF">Internal states</font></p>
<p>PTC is fundamentally a tracking code. It tracks through structures called 
&quot;layout&quot; which represent the &quot;s&quot; variable of a fibre bundle structure. The 
layouts, not surprisingly, are made out of &quot;fibres&quot;. Each fibre is itself a 
collection of &quot;integration_node&quot;.&nbsp; </p>
<p>For example, the command:</p>
<p><span style="text-transform: uppercase">CALL TRACK_NODE_probe( PROBE, 
INTERNAL_STATE, fibre1=p1,fibre2=p2) </span></p>
<p>tracks a probe from p1 to p2. A probe is an object which can contain orbital 
coordinates and spin in the present version of PTC. It also contains a certain 
object called an rf_phasor which allows for the slow rf modulation of chosen 
elements of the lattice at a unique frequency specified in the phasor.</p>
<p>The type probe is given by:</p>
<p><span style="font-variant: small-caps; text-transform: uppercase">type probe<br>
&nbsp;&nbsp;&nbsp; real(dp) x(6)<br>
&nbsp;&nbsp;&nbsp; type(spinor) s(3)<br>
&nbsp;&nbsp;&nbsp; type(rf_phasor) AC<br>
&nbsp;&nbsp;&nbsp; logical u<br>
&nbsp;&nbsp;&nbsp; type(integration_node),pointer :: lost_node<br>
end type probe</span><br>
&nbsp;</p>
<p>PTC can also track a polymorphic probe (type probe_8): this object is almost 
identical to the above probe except for the fact that it is made of real 
polymorphs rather than real(dp=8) objects. This turns potentially PTC into a 
&quot;design ca</p>
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